Now that the superpowers have tossed nuclear arms proposals at each other, the paramount question is whether they will begin serious negotiation. As US Secretary of State George P. Shultz and national security adviser Robert C. McFarlane talk with Soviet leader Mikhail Gorbachev and other officials in Moscow today and tomorrow, the superpowers appear to be inching their way toward agreeing on a statement of general arms principles at the upcoming summit. But the sides are still far apart on a substantive agreement.

``This isn't a negotiation yet,'' says Helmut Sonnenfeldt, a former key aide on the National Security Council staff. ``In firing volleys of proposals they have begun to come a little closer, but they need very intensive talks.''

Secretary Shultz -- en route to Moscow -- acknowledged that the most that could be done at the summit is to give a ``political impulse'' to the Geneva arms negotiators. This could lead to a full-scale agreement later.

The largest obstacle to an accord remains the President's unwillingness to offer any concession on ``star wars,'' the Strategic Defense Initiative (SDI) -- which the Soviets seek to curb. But the administration may be trying first to nail down parameters of an agreement on offensive nuclear weapons before playing its SDI card.

Even moderates in the arms control community voice interest in the new US proposal offered at the Geneva talks Friday. The proposal is largely a modification of previous US positions, but the differences between US and Soviet proposals are being narrowed. ``I'm still skeptical but more optimistic than I've been,'' says Thomas K. Longstreth, an expert at the Arms Control Association. ``A framework seems to be coming into place.''

According to administration officials, the US proposal embraces the general Soviet concept of a 50 percent cut.

The US proposal includes these basic elements:

A common ceiling of 4,500 warheads on land-based and sea-based missiles. (The Soviets have about 9,000 warheads and the US about 8,000). This compares with the earlier US proposal of 5,000.

The Soviet proposal calls for limiting nuclear ``charges'' -- missile warheads, air-launched cruise missiles, and bombs -- to a common ceiling of 6,000.

A limit of 3,000 on Soviet land-based warheads, which would mean a cut by more than one half of current level. This compares with the limit of 3,600 land-based warheads proposed by Moscow and 2,500 previously offered by the US.

A limit of 350 long-range bombers; the earlier US offer was 400.

A limit of 1,500 air-launched cruise missiles, or half the number projected for future production. Previously the US proposed a limit of 4,000. The Soviets propose banning all long-range cruise missiles.

A ban on deployment of mobile land-based nuclear missiles -- a position that overturns the administration's earlier decision to build the Midgetman missile as recommended in the Scowcroft Commission report in 1983. The President appointed the commission to review the US strategic modernization program.

A limit of about 140 medium-range missile launchers on each side in Europe, with British and French forces to be excluded from any agreement.

The US will soon have close to the 140 launchers, including Pershing-IIs and ground-based cruise missiles; the Soviets now have 243 SS-20s targeted on Europe. The US proposal also calls for cuts in the SS-20s deployed in Asia.

Prospects have improved for an early agreement on intermediate-range weapons in Europe, now that Moscow has signaled a willingness to separate the medium-range talks from the other two Geneva negotiations, concerning strategic weapons and space-based defenses. But arms experts believe an agreement will require the US to make some concession on its Pershing-II missiles, which can reach Moscow in minutes.

It is also believed that Moscow will not sign any accord that restrains their entire arsenal against the US but does not consider the British and French forces, that will eventually have some 1,300 nuclear warheads.

Critics of the new US proposal say the administration is still trying to restructure the Soviet strategic forces, which rely primarily on land-based missiles. ``Essentially we're telling them: either limit your force to 300 of the big ones [SS-18s] or build new smaller ones, and that they will not do,'' says Barry Blechman, an expert at the Johns Hopkins University.

John Steinbruner of the Brookings Institution agrees that success of the negotiations will hinge in part on the future of the large Soviet land-based missiles. He suggests that if the Soviets were willing to reduce their SS-18s (while the US curbs its MX missile), the offer would be so attractive the administration would have to respond.

On the US side, experts agree, there has to be some resolution of the SDI issue. The President, in announcing the US counteroffer, did not use the terms ``development'' or ``testing'' with respect to SDI research on exotic defense technologies.

This may leave the door open for eventually drawing a line between SDI testing for the purpose of technical assessment and the building of prototypes, which the Soviets oppose.